1. Field of the Invention
This invention relates to insulated power cables, particularly insulated cables commonly used in fluid-filled electrical transformers.
2. Description of Related Art
Insulated cables and insulated winding wires are both used in fluid-filled transformers. Insulated winding wire is used to form the winding of the transformer. This winding wire needs to be sufficiently stiff in order to withstand mechanical stresses that occur during operation of the transformer. Insulated cable connects various components within the transformer such as winding taps to no-load or on-load tap changers, phase interconnections, and internal windings to bushing connectors. In contrast to insulated winding wire, insulated cable needs to be sufficiently flexible to allow easy maneuverability to the connection points. The cable is then supported mechanically when additional strength is required.
The conductor of the windings in a transformer is typically composed of a number of winding wires individually insulated to prevent one wire from coming in contact with another. In many cases these insulated winding wires are rectangular in cross section to ensure a dense uniform packing of the transformer windings. In contrast, the insulated cables used in transformers are normally made from a plurality of bundles of uninsulated wires and are generally circular in cross section. Since these cables transmit electricity at high voltages and high amperages, the key requirement is that they have sufficient insulation to prevent dielectric breakdown from one cable to the next, which could be catastrophic in an oil-filled transformer. Cables in an oil-filled transformer have traditionally been insulated with spiral-wound, creped cellulosic paper tapes, and the size and number of cables used in a transformer were determined by first specifying the desired maximum temperature difference between the wire cable and the transformer oil while under load, and then using enough cables to handle the desired current without exceeding the required maximum temperature difference. For cellulosic paper tapes, the maximum temperature difference was generally about 20 degrees Celsius (Transformer Engineering, Second Ed., published by John Wiley and Sons, Page 321), because any higher temperature difference could cause premature aging of the cellulosic insulation and eventual cable failure.
However, if the cables could be operated at higher temperature, that is, if the maximum temperature difference could be increased to around 60 degrees Celsius, the size of the cables and/or the number of cables needed for the transformer could be reduced. Therefore, what is needed is a cable that can withstand a higher temperature without premature aging of the insulation.
Research Disclosure RD10833, April 1973 discloses wire conductors can be wrapped using a “longitudinal-wrapping” technique wherein a narrow tape of Nomex® is applied parallel to the wire, folded around the wire, and sealed. It is preferred to use a tape that had been creped and then lightly calendered to maintain a desirable thickness for the insulation.
Research Disclosure RD10947, May 1973 discloses that in certain applications where high porosity is desired, such as insulation for oil-filled transformers, a special low density paper, e.g. Nomex® 411 is particularly preferred.
WO200191135 to Rolling et al. discloses an electrical apparatus that includes one conductor and an insulation paper surrounding at least part of the conductor; the insulation paper includes a wood pulp fiber, a synthetic fiber which can be an aramid fiber, and a binder material, with the synthetic fiber being present at between 2 and 25 weight percent. The insulation paper can be creped and spirally wrapped around the conductor.